1. Field of the Invention
This invention pertains to protection devices for a starting circuit of a high intensity gaseous discharge lamp, especially of the type of starter employed with a high pressure sodium lamp.
2. Description of the Prior Art
Starter circuits for high intensity, gaseous discharge lamps of the high pressure sodium type produce extremely high voltage pulses to initiate the lighting of these lamps. That is, the starter circuits are purposely designed to produce such pulses to an unlit lamp when power is applied to the circuit. Such lamps however, are not foolproof in design. When they are operating in the lighted condition, occasionally an open connection occurs either at the terminals of the lamp or in the connections to the lamp. When this happens, because of the inherent design of the starter circuit, there is almost certain to be such a high voltage applied back into the starter circuit components so as to cause failure of one or more of these components. That is, there will exist a momentary high voltage of such high value that a capacitor, a resistor, a triac, a transformer or more than one of such components will fail. Although it is possible to include fuses or circuit breakers, such conventional devices do not respond quickly enough to accomplish the protection required.
Therefore, it is a feature of the present invention to provide an improved starting and protection means for a high intensity, gaseous discharge lamp of the type that requires high voltage pulses to be applied to the lamp when it is in its unlit condition.
It is another feature of the present invention to provide a protection element for a circuit receiving extremely high voltage pulses which has a negative coefficient of resistance only at extremely high voltages and therefore functions as a normal resistor except in the presence of such high voltage.
It is still another feature of the present invention to provide a protection element for high voltages that operates even in the presence of extremely short pulses.